Old time sailors say that the time to reef sails (reduce sail area) is when you first think about it. A similar statement can apply to tire studs.

Assuming you have a real finger on the weather's pulse and can get it down to the very day that the odds of hitting ice are 50/50. So you have a choice of studs or no studs, and whatever you decide have a 50/50 chance that's the right decision. That's the bright side, but you also have a 50/50 chance of being wrong, which is where things get interesting.

If you mount the studs when you didn't need them, all that happens is you have a bit slower ride, and some stud wear. Probably you won't even think about it much.

OTOH- what if you should have mounted the studs but didn't. So you hit a sheet of ice and the bike slide out from under you. At the least you bruise your arm and shoulder, at the worst.....

So if you're going to be wrong, which error is preferable?

The time to mount the studded tires is when you first think you might need them.